Optical fiber with enhanced light collection and illumination and having highly controlled emission and acceptance patterns

ABSTRACT

An optical fiber, including a cylindrical core of light conducting material having a first refractive index, surrounded by a cladding having a second refractive index that is lower than the first refractive index. The optical fiber includes at least one endface formed with a cone tip for controlling the characteristics of light entering or exiting the fiber. The polish angle of the cone tip can be selected according to the desired emergence/acceptance properties of the fiber.

This is a division of application Ser. No. 08/561,484, filed Nov. 20,1995 now U.S. Pat. No. 5,164,840.

TECHNICAL FIELD

This invention relates generally to optical fibers, and moreparticularly to optical fibers having at least one endface provided witha cone tip for controlling the pattern and characteristics of lightentering or exiting the fiber.

BACKGROUND OF THE INVENTION

In recent years, the use of optical fibers has become increasinglywidespread in a variety of applications. For example, high capacityoptical fibers are rapidly replacing traditional telephone and coaxialcable lines to transmit voice and video signals across the country.Fiber optic sensors and probes are also commonly used by physicians andresearchers for such things as the treatment of cancer and for spectralanalysis of samples.

The term "optical fiber" is used herein to refer generally to anyoptical waveguide or structure having the ability to transmit the flowof radiant energy along a path parallel to its axis and to contain theenergy within or adjacent to its surface. The optical fiber categoryincludes both "step index" and "gradient index" fibers. The term"multimode" optical fiber refers to an optical waveguide that will allowmore than one bound mode to propagate. This property is typicallyachieved in fibers whose core diameter is relatively large (typically atleast equaling about 10 microns) compared with the wavelength of theluminous radiation carried by the fiber. In contrast, "single mode"optical fibers have a core diameter on the order of magnitude of thewavelength, generally only a few microns.

In the general sense, numerical aperture (NA) refers to the vertex angleof the largest cone of meridional rays that can enter or leave anoptical system or element, multiplied by the refractive index of themedium in which the vertex of the cone is located. In the context offiber optics, numerical aperture is used to refer to the lightacceptance or emergence characteristics and is a measure of lightgathering ability. Numerical aperture value is often used tocharacterize bare, unterminated fiber, and in this context, it specifiesthe characteristics of the fiber with the ends polished flat. Thus,numerical aperture for conventional optical fibers has often beendefined as:

    NA=n.sub.2 sinθ.sub.e

where n₂ is the refractive index of the medium (typically air) throughwhich the light is initially propagated so as to be incident upon aninput end of the fiber, and θ_(e) is one half the included acceptanceangle (or, conversely, the emergence angle) at the input end of thatfiber. The "acceptance angle" of a fiber refers to the angles withinwhich the input end of the fiber will accept a cone of light and undergototal internal reflection, while the "emergence angle" corresponds tothe illumination pattern of light that it emitted from the output end ofthe fully filled fiber. Thus, an acceptance angle of θ_(e) indicatesthat the fiber will accept a cone of light within ±θ_(e). The greaterthe acceptance angle, the larger the light gathering ability of theoptical fiber. Similarly, the fully filled fiber will have anillumination pattern defined by these angular limitations.

Numerical aperture may also be defined as a function of the physical(optical) properties of the fiber's materials of construction: ##EQU1##where n₀ is the refractive index of the fiber core, and n₁ is therefractive index of the cladding (the medium cylindrically encirclingthe core). Thus, conventional flat-faced optical fibers have numericalapertures which are primarily a function of the refractive indices ofthe core, cladding and media surrounding the endface.

Depending on the particular application, it may be preferable to have anoptical fiber with larger or smaller angles of acceptance and emergence.For example, in certain sensing applications, it may be desirable to usean optical fiber with a relatively large acceptance angle so that thefiber will collect light more efficiently from the sample beingmeasured. Similarly, when an optical fiber is used for illumination orindicating purposes, it is often advantageous for the light to emergefrom the optical fiber with a large illumination pattern so that thelight will be visible from wide viewing angles.

On the other hand, for many other applications, it may be desirable tominimize the acceptance and emergence angle of the optical fiber. Forexample, a fiber optic probe commonly includes at least one transmittingoptical fiber that emits light into a sample to be measured and at leastone adjacent receiving optical fiber that receives light reflected fromthe sample. By measuring the light scattered by the sample and comparingit to the source light, certain characteristics of the sample can bedetermined. In these cases, it is undesirable for light to pass from thetransmitting fiber directly to the receiving fiber without firstinteracting with the sample to be measured. This criterion is difficultto meet when the fibers are positioned behind a window. To minimize thiseffect, it may be preferable for the transmitting fiber to have aspecialized emergence pattern which projects its energy through thewindow and outward into the sample before rapidly diverging, andlikewise for the receiving fiber.

Traditionally, the pattern and characteristics of light entering orexiting the fiber was controlled by selecting a combination of core,cladding and surrounding media such that the numerical aperture oracceptance/emergence angles were suitable for the specific application.However, there are significant physical limitations with this methodthat affect the ability to adjust the numerical aperture of thoseoptical fibers. As a result, conventional flat-faced optical fibers havea relatively narrow acceptance angle, so that these fibers have poorlight gathering characteristics and can often collect only a smallfraction of the available light. This is particularly problematic in thecase where the light beam is incident on the fiber face from extreme andhighly diverse angles as the optical fiber can only accept those lightrays that arrive at an angle less than or equal to its acceptance angle.

A variety of different solutions have been proposed in attempting toimprove the light gathering ability of flat-faced optical fibers. Manyproposals utilize discrete optical elements in front of the fiber face,such as lenses, prisms, mirrors, etc. in order to enlarge the acceptanceangle of the optical fiber. However, this adds significantly to thecomplexity and the cost of the device, and the resulting fiber assemblyis bulky and not mechanically robust. The optical elements are alsoinherently prone to misalignment, shifting, stresses, shock, cracks,scratches, etc.

In attempts to improve upon the displacement sensitivity in laser diodesource-to-fiber coupling of conventional flat-faced optical fibers, thefiber faces of some single mode fibers have been formed into variousshaped surfaces, usually spherically shaped. However, these fibers lackcontrol of light acceptance characteristics, are constrained by the sizeof the fiber, are limited in their ability to collect light at wideangles, and are not effective at preventing light reflected off theendface from back propagating toward the light source.

Thus, there is a need for an improved optical fiber that provides bettercontrol of the pattern and characteristics of light entering or exitingthe fiber.

There is also a need for an improved optical fiber that provides betterlight gathering ability, without the need for expensive and complexoptical elements.

SUMMARY OF THE INVENTION

As will be seen, the foregoing invention satisfies the foregoing needsand accomplishes additional objectives. Briefly described, the presentinvention provides an optical fiber including a cylindrical coresurrounded by a cladding layer. The core comprises a light conductingmaterial such as glass, silica, plastic or quartz. The claddinggenerally comprises a light conducting material with a refractive indexthat is lower than the refractive index of the core. The cladding mayalso comprise air or other gas, so long as it has a lower index ofrefraction than that of the core.

The optical fiber includes at least one endface formed with a cone tip.If the optical fiber is utilized to receive light, the cone tip of thefiber defines an acceptance angle for receiving light incident upon thefiber's endface. Conversely, if the optical fiber is being used foremitting light, the cone tip defines an emergence angle for emitting acone of light. The acceptance and emergence angles are a function of thepolish angle of the cone tip. Therefore, by selecting an appropriatepolish angle, the characteristics of the light entering or exiting theoptical fiber can be controlled to a greater degree than withconventional flat-faced optical fibers.

The tip of the coned-shaped endface may be positioned along the centrallongitudinal axis of the optical fiber, or it may be offset from center.

According to another aspect of the present invention, the optical fiberis housed within a transparent window positioned across the cone tip. Inthis manner, the window forms a chamber between the inside of the windowand the cone tip. The window provides protection for the optical fiberby physically shielding the core from any hostile effects from theenvironment in which the fiber is being used. In addition, the chambercan be filled with air or other gas with a known index of refraction.This allows the behavior of light emerging from the fiber to moreaccurately predicted and controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an optical fiber with a cone tipaccording to a preferred embodiment of the present invention;

FIG. 2 is an end cross-sectional view of the optical fiber shown in FIG.1, illustrating meridional and nonmeridional planes and the path of anonmeridional skew ray.

FIG. 3 is a side cross-sectional view of the optical fiber shown in FIG.1, along the described meridional plane illustrating the path of ameridional ray.

FIG. 4 is a side cross-sectional view of the optical fiber shown in FIG.1, along the described nonmeridional plane illustrating the path of anonmeridional skew ray.

FIG. 5 is a side cross-sectional view of the right side of thetriangular conical section of the fiber tip along a meridional plane.

FIG. 6 is a perspective view of an optical fiber with a cone tip havingits center offset from the center longitudinal axis of the fiberaccording to an alternative embodiment of the present invention.

FIG. 7 is a plan section view of an optical fiber with a cone-shaped tiphoused within a window assembly, and depicting the pattern of limitinglight rays emerging from the cone-shaped tip as compared to the patternof rays emerging from a flat-faced fiber.

FIG. 8 is a side cross-sectional view of an optical assembly accordingto a preferred embodiment of the present invention, including an opticalfiber having a cone-shaped endface that is coupled to a flat-facedoptical fiber.

DETAILED DESCRIPTION

Referring now to the drawing figures, in which like numerals indicatelike elements or steps throughout the several views, the preferredembodiment of the present invention will be described. FIGS. 1-4 depicta step-index, multimode optical fiber 9 constructed in accordance withthe present invention. In particular, the optical fiber 9 includes atleast one of its endfaces 10 provided with a cone-shaped tip. With theexception of the endface, the optical fiber 9 is of a conventionalstructure and includes a transparent cylindrical core 3 of a relativelyhigh refractive index, such as glass, silica, plastic, quartz or otherlight-conducting core material. The core 3 is surrounded by a relativelythin cladding 2 of lower refractive index material which is intimatelybonded to core 3. Cladding 2 may be surrounded by an outer coat orbuffer 1 to protect the fiber against damage. Although FIG. 1illustrates an integral cladding layer 2 and an outer buffer 1, thoseskilled in the art will appreciate that the fiber 9 may also have anunclad core 3 with the surrounding media (often air) having a lowerindex of refraction than the core.

In the preferred embodiment, the tip 4 of the fiber's cone-shaped face10 is centered in the fiber's core 3. However, the tip 4 may also beoffset from the fiber's center longitudinal axis, for example, as shownin FIG. 6.

Still referring to FIGS. 1-4, the optical characteristics and basicdesign criteria of the optical fiber 9 with a cone tip will be describedin detail. For ease of explanation, the following description is basedupon a ray analysis rather than a vector analysis or full wave analysis.It will be appreciated that the following analysis does not account forphysical effects such as fiber length, physical imperfections and launchconditions. In addition, the near field effects may be significant whenthe device is utilized in close proximity to additional opticalcomponents. These effects are ignored for purposes of this description.

Referring to FIGS. 3 and 4, an arbitrary ray 15 exiting the fiber at apoint 5 on the cone-shaped fiber face 10 radially offset from thecentral axis of the fiber's core 3 will be analyzed. The polish angle ofthe cone face 10 is φ (see FIG. 3). Although this analysis assumes anemerging ray, the system is primarily symmetric with respect to lightentering the fiber. Therefore, the results are equally valid for andcorrelated to light entering or exiting the fiber. Further, even thoughonly a single point on the fiber face is analyzed, the analysis is validfor any and all points on the fiber's face with the exception of the tip4. For the purposes of this analysis, however, the tip 4 is assumed tobe an infinitely small point. Thus although the tip of the physicalembodiment has some physical size, it remains negligible for purposes ofthis analysis.

The emerging ray 15 may be oriented in any direction which the fiber iscapable of transmitting via total internal reflection. As is well knownin the art, for total internal reflection, the ray's angle of travelwithin the fiber core 3 must be smaller than the critical angle θ_(c) ofthe fiber. The critical angle θ_(c) is defined by the relationship:##EQU2## where n is the refractive index and the angle is referenced tothe fiber's longitudinal axis. This relationship defines the angularlimits within which the fiber can conduct rays within the fiber core 3.It also dictates the population of angles of light rays within a fiberof sufficient fiber length, launch conditions, fiber stress, etc. toachieve equilibrium model distribution. The corresponding limits outsidethe fiber will be discussed below.

As shown in FIG. 2, the emerging ray under analysis can be split intotwo component rays. The first ray 12 (FIG. 3) lies in a meridional plane6 that intersects the fiber core 3, and which is defined by inclusion ofthe point of analysis 5 with the fiber's center longitudinal axiscontaining the tip 4. The second ray 14 (FIG. 4) lies in a second,nonmeridional plane 7 perpendicular to the meridional plane whichintersects the point of analysis 5, and is parallel with the fiber'scenter longitudinal axis.

Because this example assumes that the tip 4 of the cone face is in thecenter of the endface 10, the fiber is cylindrically symmetrical aboutthe cone's central longitudinal axis. Since all points on the fiber face10 are radially displaced from the fiber's central axis, the twocoordinate planes can be established for any point. The meridional plane6 containing the point of analysis 5 is established so as to intersectthe fiber's center. The nonmeridional plane 7, perpendicular to themeridional plane 6 and also containing the point of analysis 5, isalways tangent to a cylindrical surface whose center axis coincides withthe fiber's center longitudinal axis and whose radius equals the radialdisplacement of the analytical point from the fiber's center.

FIG. 3 illustrates the path of the meridional component ray 12, whichlies in the meridional plane 6. In the physical implementation, themeridional component ray 12 is typically the strongest of the twocomponent rays and is oriented in the general direction of light travel.

FIG. 4 illustrates the path of the nonmeridional component ray 14, whichlies in the nonmeridional plane 7. The nonmeridional component ray 14,also referred to as the skew ray, traces a rotational path 8 around thelongitudinal axis of the fiber as the ray traverses the fiber (see FIG.2).

The nonmeridional plane 7 forms, or sections, a hyperbola 13 as itintersects the surface cone of the fiber's face 10, as shown in FIG. 4.For a given polish angle φ, the shape of the hyperbola 13 is dependentupon the axial displacement of the point of analysis 5 from the fiber'scentral longitudinal axis. Regardless of this factor, the skew ray 14exits the fiber face at the vertex of the hyperbola 13. At this point,the hyperbola is flat or has zero slope; and, as the ray exits thefiber, it is refracted accordingly. Since the skew ray 14 lies fullywithin the plane of analysis and exits the fiber at a point with zeroslope within the plane of analysis, it reacts to the surface as if itwere flat. In other words, the skew ray 14 remains unaffected by thefiber's cone shape.

For the physical embodiment, the actual density of light traveling in askew path is determined by factors such as the characteristics of thelight source (not shown), launch conditions, length of the opticalfiber, and fiber twist. The physical skew ray density may also not beradially uniform in the fiber. For most applications, however, the skewray component will be minor.

Although the skew rays 14 have been shown to be generally immune toinfluences of the nonplanar surface of the cone-shaped endface 10 of thefiber, the geometry of the cone tip allows the meridional rays 12 to beprecisely manipulated, as will be demonstrated.

As previously described, the angular limits within which the fiber canconduct rays within the fiber core 3 is defined by the equation:##EQU3## Outside the fiber core 3, the corresponding limits aredifferent due to refraction as the rays cross the boundary between fiberand external media. Whether entering or exiting the fiber, the rays arerefracted according to Snell's law (n₂ sin I₂ =n₃ sinI₃) as they crossthe boundary between the two media of differing refractive indices,where n represents the refractive index and I represents the angle ofray incidence with respect to the surface normal.

Building on these relationships, an acceptance angle for a conventionalflat-faced fiber can be established according to the following equation:##EQU4## The term on the left hand side of the equation is oftenreferred to as the numerical aperture of the fiber. The external angleθe corresponds to the critical internal angle θc. Thus, a fully filledfiber has an illumination pattern of a cone defined by ±θe. Similarly,the fiber accepts a cone of light within these angular limitations. Raysentering the fiber at angles beyond θe do not undergo total internalreflection at the core/cladding interface within the fiber. These raysare eventually lost through the cladding since each core-to-claddingreflection is less than 100% efficient.

In contrast to a conventional flat-faced optical fiber, the cone-shapedendface of the present invention significantly changes theillumination/acceptance characteristics of the fiber. The cone-shapedtip facilitates efficient acceptance of light and increases lightgathering ability. More importantly, the optical characteristics andzones of acceptance, attenuation, and rejection of an optical fiber witha cone tip can be controlled to a greater degree than with conventionalflat-faced optical fibers, as will be fully described below inconnection with FIG. 5. Likewise, the illumination pattern can becontrolled to significant advantage.

FIG. 5 depicts the right hand side of the triangular conical section ofthe fiber tip. As previously described, and within the limitationsstated, the primary, meridional component of any ray within the core 3can be analyzed with a corresponding meridional plane 6. Furthermore,the meridional component ray 12 is fully contained within the meridionalplane 6. As the meridional plane 6 intersects the cone endface 10, atriangular shape is generated or sectioned. The inclination of thetriangle sides remains constant despite the radial offset of the rayfrom the fiber center. The angles of the triangle are fully defined by,and equal to, the polish angle of the cone of the fiber endface φ.

Due to the cylindrical symmetry of the fiber and the nature of thecoordinate planes established, a single analytical point can be studiedto fully characterize the optical behavior of the fiber's cone-shapedendface.

In the preferred embodiment, the relationship between the angles ofexternal and meridional rays are governed by the following equation:

    n.sub.core sin(θ+φ)=n.sub.2 sin(φ+α)

where θ refers to the angle of the ray with respect to a unit vector(A_(FI)) that is parallel to the fiber's longitudinal axis andpositioned at the point of analysis; φ refers to the polish angle of thefiber cone endface; n₂ is the refractive index of the surrounding media(air≈1); and α refers to the angle of the refracted ray R_(nE) withrespect to the unit vector A_(FI).

As the emerging ray intersects the internal cone surface, an internalreflected ray R_(nR) is also generated. The angle of this ray is definedaccording to the relationship:

    ρ=-θ-2φ

where ρ is the angle of the internally reflected ray R_(nR) with respectto the unit vector A_(FI). Thus, those skilled in the art willappreciate that it is possible to select the polish angle of the fibertip ρ to control whether or not the internally reflected ray will bewaveguided back towards the light source (not shown).

To illustrate how the cone-shaped surface of the optical fiber can beutilized to control the optical illumination/emergence characteristicsof the fiber, the meridional ray shown in FIG. 5 will be described andanalyzed in three different cases: maximum positive angle, maximumnegative angle and average (zero angle). Characterization of the threecases are undertaken separately. For the purposes of this example, θ_(C)<φ, NA=0.22, n₁ ≈1 (air), n_(cladding) =1.46 (silica), φ=20°, andθc=8.5°. For clarity, the three rays have been displaced to separate,but equivalent, analytical locations.

The first ray R1 19 is incident to the cone-shaped surface parallel tothe fiber's longitudinal axis (θ=0°). The second ray R2 20 is directedoutward from the fiber's longitudinal axis by the maximum allowableangle θ_(c) (θ=-θ_(c)). The third ray R3 21 is directed inward towardsthe fiber's longitudinal axis by the same angle θ_(c) (θ=θ_(c)). Thefiber's symmetry allows the results to be applied to the left hand sideof the triangle. Furthermore, the results are readily applied to allmeridional rays within the fiber.

In a conventional flat-faced fiber, a ray that is parallel to thefiber's longitudinal axis would emerge from the fiber normal to thefiber face. However, in the preferred embodiment, the first ray 19 isdirected inward toward the longitudinal axis of the fiber by thecone-shaped fiber face. The amount of refraction is dependent upon theangle of polish φ and the refractive indices of the core and surroundingmedia. As long as the core material has a higher refractive index thanthe surrounding media, the refraction is inward. For steep cone angles(large φ), this ray undergoes total internal reflection at the boundary.Depending on φ, n_(core), n₁, and θ_(c), this ray may be forced tomultiple total internal reflections and redirection with the innersurface of the cone.

In the second example shown in FIG. 5, the second ray 20 continues todiverge away from the longitudinal axis of the fiber as it emerges fromthe cone shaped fiber face. However, the divergence angle is less thanwhat occurs for a flat-faced fiber. For a properly chosen polish angleφ, the emerging ray may be forced to be parallel to the fiber's axis.(α=0°).

In the third example shown in FIG. 5, the third ray 21 is directedinward towards the fiber's longitudinal axis, but at a steeper anglethan would occur for a conventional flat-faced fiber. For a properlychosen polish angle φ, the emerging ray may be forced to undergo totalinternal reflection at the core boundary.

Thus, it will be appreciated that the optical fiber constructed inaccordance with present invention allows for improved control of theillumination/emergence characteristics than conventional flat-facedoptical fibers. For example, the endface of a source fiber can be shapedin such a manner to prevent energy from back reflecting off the endfacesurface towards the source.

The above analysis assumed that the tip of the fiber's cone face wascentered along the central longitudinal axis of the core. However, theadvantages of the cone-shaped face may also be utilized if the tip ofthe cone face is offset from the central longitudinal axis. FIG. 6illustrates an example of one such optical fiber 29, with the tip 4 ofthe fiber offset from the center of the fiber.

In the preferred embodiment, the cone tip fibers are produced withstandard fiber processing equipment adapted for the fabricationprocedure. The fiber polishing equipment is preferably the variety withrotating abrasive disk platens. A holding mechanism, such as a collet,chuck, or similar device, is required to support and position the fiberfor polishing. The holding mechanism must maintain the fiber's primaryaxis at the desired angle of polish relative to the rotating disk.Whereas conventional, flat-faced fiber polishing is accomplished bypositioning the fiber's central axis at a 90° angle relative to thesurface plane of the rotating disk, the pointed tip fibers of thepresent invention are formed by positioning the fiber at a lesser angle.

The holding mechanism also preferably includes a provision tosimultaneously rotate the fiber about its major axis and sweep it backand forth across the abrasive disk. It is important for the holdingmechanism to possess sufficient precision so the axis of rotation isaccurately maintained with respect to the fiber's center longitudinalmechanical and optical axis.

To form the point on the fiber's endface, the fiber should becontinuously rotated as it is swept back and forth across the polishingdisk. Progressively finer polishing media are used to create a highlypolished surface.

Depending on the desired optical effect, the tip of the pointed fibermay be further formed to create additional light-shapingcharacteristics. For example, the tip may be formed with compound anglesor with a flat end so the endface is a frustum of a cone.

As shown in FIG. 7, the optical fiber 9 may be housed within a windowassembly, including a transparent window 25 displaced across the fibertip 4 and a housing 26. The window 25, which is preferably made ofsapphire or silica, protects the fiber tip 4 from being damaged by theenvironment in which it is being used. The fiber 9 is mounted within atermination ferrule 27, which is often referred to a fiber optictermination connector. The window 25 is connected to the fiber 9 byhousing 26, typically a cylindrical metal tube, that is epoxied totermination ferrule 27 to secure the fiber in position. In addition toprotecting the fiber, the window 25 provides the advantage of defining aclosed chamber 28 between the inside of the window 25 and the opticalfiber 9. By filling the chamber 28 with air or other gas having a knownindex of refraction, the behavior of light emerging from the fiber 9 canbe more accurately predicted and controlled.

FIG. 7 also illustrates the pattern of light rays emerging (solid lines)emerging from the cone-shaped fiber 9. To demonstrate the physical(optical) effect of the cone-shaped endface, the pattern of lightemerging from a conventional flat-faced optical fiber is shown in dashedlines.

In certain applications, a window 25 is placed in front of a sourcefiber adjacent to one or more receiving fibers. If the source fiber is aconventional flat-faced fiber, energy may be reflected off the window 25and directed into the adjacent components. However, in the preferredembodiment, by angling the surface of the source fiber's cone, such thatemerging rays are directed either converging or parallel to the fiber'scenter axis, back reflection from the window's surface 25 is directedback into the source fiber and thus prevented from interfering withadjacent components. Likewise, receiving fibers may be formed to rejectluminous energy from adjacent source components.

As previously described, the cone-shaped tip controls light emergenceand acceptance from the optical fiber. The effect is based on the polishangle of the cone tip and the refractive index change between the coreand the media surrounding the fiber tip. For a typical application, thesurrounding media may be air or similar gas (with a refractive indexapproximately equal to one). However, a substantial index change isachieved with a relatively high index media, such as liquids, byselecting fibers whose cores have high indices of refraction (such assapphire or diamond) and/or steep cone angles.

It is also possible to use a cone-shaped fiber in conjunction with aconventional flat-faced fiber to achieve similar light control on anyfiber. By this method, constraints placed on the shape of the distal tipare eliminated. Therefore, light control is achieved on fibers whosedistal tips are flat. Further, by forming the distal tip as a complexsurface such as angled planar, cone-shaped, spherical, cylindrical.etc., further control effects are realized. Constraints at the distaltip related to differing indices of refraction between the fiber coreand the surrounding media are also relaxed or eliminated.

In other words, light control similar to that of a cone-shaped fiber tipsurrounded by air may be achieved with any tip shape, in any medium byusing an optical fiber assembly comprised of a section of optical fiberwith a cone-shaped endface and an adjoining section of a flat-facedoptical fiber.

Such an arrangement provides several advantages. For example, theresulting optical assembly does not have a fragile end-point that issusceptible to damage, thereby reducing the need for a window. Further,the optical assembly is not dependent on the change in refractive indexbetween the fiber and surrounding media. The assembly can also befabricated in a diametrically small package.

FIG. 8 illustrates one example of an optical assembly 30 having oneoptical fiber 35 having a cone-shaped endface 40 that is butted up to asecond optical fiber 45. The distal end 50 of the second optical fiber45 is generally flat-faced but may also be cone-shaped, plane-angled,spherically-shaped, mated surfaces, etc. Anti-reflective coatings can beapplied to optical surfaces so as to maximize transmission.

The connection 60 between the first optical fiber 35 and the secondoptical fiber 45 is achieved with standard fiber optic matingconnectors. Alternatively, miniature sleeves may be employed, such ascapillary tubing and hypodermic tubing. Depending on the application,various couplers and industry-standard mechanical splices are suitable.These splices include those known in the industry such as elastomeric,capillary tubing, V-groove, etc.

Various materials (fluids, gels, epoxies, etc.) can be applied withinthe open space of the cone-faced fiber to flat-faced fiber junction 60.The refractive index of the material will determine light shapingcharacteristics. For media with higher indices of refraction than thecore, the light bending will still follow the equation and therefraction will be opposite that observed for lower indices.

If the optical assembly 30 will be used in a manner such that theconnection 60 will be subjected to rough handling or repeatedconnect/disconnect cycles, a special "stand off" can be permanentlyattached. Subsequent to forming the cone-shaped endface 40, athin-walled, rigid tubular sleeve is positioned over the tip and epoxiedin place. Although the sleeve end is typically precisely flush with thefiber face's cone tip so as to maximize power transfer, the tip may berecessed relative to the sleeve end. The sleeve facilitates precisepositioning of the cone-shaped fiber tip 40 against the flat-faced fiber45 or other surface. Mechanical stress on the cone tip 40 is minimizedso as to prevent breakage and optical effects related to stress.

Two techniques are useful depending on the type of epoxy and desired endresults. In the first method, the end of the sleeve should be cut orground perpendicular with its lengthwise axis and free from burrs. Thesleeve is inserted over the connector-mounted or bare fiber tip so thatit is free to slide. The sleeve is temporarily moved up the fiber andaway from the tip. The fiber is positioned perpendicular to a plate ofglass, or other flat surface, by holding it from above. The fiber islowered until the cone tip touches the glass plate or is positioned atthe desired distance above it. When the desired position is achieved,the sleeve is permanently attached with epoxy or similar bonding agent.Care must be taken so as not to contaminate the endface with epoxy.

In the second method, the sleeve is permanently attached so it extendsbeyond the desired stand off length. The assembly is ground down withstandard optical fiber polishing methods to the desired stand offdistance (typically precisely flush). The fiber point may be coated withremovable dye or similar material so as to provide a visual indicator asto when a flush polish is achieved.

For mating bundles of optical fibers, the individual fibers within thebundle may be aligned at the connection by starting with a bundle offibers whose continuous length is the desired length of the overallassembly. The bundle is tightly constrained and epoxied/bonded in theregion in which the connection is to be formed. Tubing is a suitablecomponent to achieve this goal. Heat shrink tubing is desirable becauseof its ability to tightly constrain the fiber while allowing removalfollowing epoxy cure and assembly. Rigid tubing such as metal, glass, orceramic may also be used. Industry-standard connectors may also besuitable. The fact that this type of tubing is permanent may be abenefit or hindrance depending on the desired characteristics of thefinal assembly. If the connection location is far removed from the end,care must be taken so as not to damage the fibers as they are insertedinto the tubing.

After the region is constrained and epoxied/bonded into a rigid section,a mechanical key or other identifying mark should be placed along thesection parallel to the axis of the fiber. The section is then cutperpendicular to the fiber axis. The cut is best achieved with a thinprecision saw such as a fine grain, diamond impregnated wheel. Afterprocessing and polishing each side of the cut to form the appropriatesurfaces, the individual fibers are realigned by mating the two bundlestogether. This mating connection can be accomplished by any of thepreviously described methods for single fibers. Rotational alignment canbe achieved by visually matching the previously described identifyingmarks or with the mechanical keys.

Referring to FIG. 8, the optical characteristics of the optical assembly30 will be described. The following discussion assumes the perspectiveof an emerging ray 55 exiting the cone-shaped tip 40 and propagatingthrough the second optical fiber 45. However, the analysis is equallyvalid for incoming rays and the correlation is readily drawn. Asdiscussed above, the optical performance characteristics achievedrelated to ray acceptance patterns are particularly relevant in sensingapplications. In the preferred embodiment, light acceptance (capture ofincoming rays) is the primary intended mode of operation for the opticalassembly 30. The following analysis ignores the near field effects atthe cone-faced fiber to flat-faced fiber junction 60, which may causethe actual results to deviate slightly from theoretical predictions.

An arbitrary light ray 55 in a meridional plane propagates in the first(proximal) fiber section 35 towards the distal endface 50 of the second(distal) fiber section 45. The light ray 55 is incident upon thecone-shaped endface 40 at an angle θ₁. The cone-shaped endface causesthe ray to refract at angle θ₂ as it propagates in the gap in theconnection 60 between the first fiber 35 and the second (distal) fibersection 45, according to the previously described relationships. Itpropagates through the surrounding media at an angle θ₂ until it isincident on the adjoining fiber face 65, where it is refracted to a newangle θ₃.

The ray 55 propagates in the second fiber 45 at the new angle θ₃ untilit is incident on the core boundary. If the angle is beyond the fiber'scritical angle for total internal reflection, a portion of the ray istransmitted into the media surrounding the core 3 (typically thecladding 2). The amount of loss is dependent upon the refractive indexof the media and the angle of incidence. For angles slightly beyond thefiber's critical angle, the loss is typically insignificant for severalreflections. For larger angles, the loss will be extreme.

A light ray 70 may escape the cladding 2 or the ray can be held withinthe bounds of the fiber by metallizing the fiber's surface forreflectivity. The metallization, such as aluminum or gold, can beapplied to the core 3 or the outside cladding 2. For silica core/silicaclad fiber, the metallization is applied directly to the fiber cladding3 (the buffer or other coating (not shown) must first be removed). Inthis configuration, the portion of the ray escaping the core isrefracted as it enters the cladding and is reflected by the metal so asto re-enter the core as a separate ray 75. Over long fiber lengths, thelosses become severe due to reflection losses and propagation within thecladding section. Even highly reflective surfaces such as aluminum haveless than 100% reflectively, and repeated reflections quickly attenuatethe ray.

The ray 55 emerges from the fiber's distal face 50 and is refracted asit enters the surrounding media. It propagates in this medium at anangle θ₄ influenced by the refractive index of the medium. For the samemedium at the distal tip (equal refractive indices), as in a connectionbetween a cone-shaped endface and a flat tip, the absolute value of theangle θ₄ of final emergence equals the angle of the ray θ₂ from thefirst cone-shaped endface (ignoring minor and long-term effects). Theemergence angle is positive or negative according to the number ofbounces during propagation.

Accordingly, when light is launched into the proximal section 35 and ispropagating toward the cone-shaped tip 40, the special connection 60fills the distal section 45 with higher order modes than are present inthe proximal section 35.

This characteristic is useful for sources such as lasers which underfillfibers due to their highly directional characteristics. Thus the opticalassembly 30 is utilized to create equilibrium modal distribution withinan optical fiber. Fibers serving this purpose are generally referred toas `launch or launching fibers`. Conventional launching fibers oftenhave portions subjected to microbending stresses or wound in a tightbending loops. A launch fiber couples and conditions light from a sourceinto an optical fiber. In this configuration, the proximal section willtypically be short. The cone angle is minor so as not to overfill thedistal section.

For illumination applications, the distal section 45 will typically bevery short so as to minimize losses through the fiber sides. The distalsection 45 will be overfilled with light rays whose angles ofpropagation are beyond the fiber's normal limits for total internalreflection. As a result, the exit cone of light emerging from the distalendface 50 diverges more rapidly than otherwise possible. Metallizingthe distal section 45 tends to increase the percentage of lightdelivered through the distal endface 50. And, the metallization allowsfor steeper ray angles to be propagated, resulting in wider angles ofbeam divergence at the distal endface 50.

For certain applications, the characteristics of the overfilled rays toescape through the fiber side can be used to advantage. Thus,cylindrically emanating light is generated. The effect is heightened byredirecting or back reflecting the light at the distal endface 50 backtoward the light source (not shown). In this case, a cone-shaped endfacecan be applied to the distal endface so that any rays reaching thisendface are reflected back at an extreme angle to further encourageescape through the fiber's side walls.

By forming the cone on the end of the fiber such that the point isoffset from the fiber's central axis, the illumination pattern emergingfrom the distal endface 50 is directed as required. Graphical rayanalysis facilitates determining the appropriate geometry to achieve thedesired illumination pattern. Factors affecting the illumination patterninclude fiber diameter, propagation angles within the proximal section,cone angle, length of distal section, and refractive indices ofmaterials involved. In the preferred embodiment, the distal section 55is kept as short as possible so as to minimize mixing effects which tendto scramble the light and convolute the emerging illumination pattern.

The optical assembly 30 is also valuable in its ability to controlreceptivity of a fiber. Incoming rays which are normally lost through afiber's side are directed to angles within the normal limits ofpropagation for total internal reflection. By keeping the distal section45 short, the losses are minimized prior to refraction to angles withinlimits for normal propagation via total internal reflection. Thus, theacceptance cone of the fiber is broadened or directed as dictated byspecific application requirements.

Analogous correlations are readily drawn between illumination andacceptance patterns. Illumination patterns are utilized in conjunctionwith ray analysis as a design aid to realize specific ray acceptancecharacteristics.

Optical filters such as band pass, high pass, low pass, and band stopfilters can be applied in the connector 60 between the distal 45 andproximal 35 fiber sections. The filters can be wafers which are insertedinto the junction or directly applied as coatings to the fiber faces.Care must be taken in applying interference filters as filtercharacteristics are related to ray angle through the filter. For bestresults, interference filters should be oriented with respect to thecone shaped endface 40 so that ray bending via refraction has alreadyoccurred. In other words, the ray angle should as close to normal(perpendicular) as possible to the filter surface. For example, whenfiltering received light propagating from the distal endface 50 towardthe proximal end 65, the cone-shaped endface 40 is best formed on thedistal section of fiber. It should be noted that this configuration canbe used to filter while minimizing back reflection returning to thelight source (not shown).

It is possible to employ other light shaping endfaces to achieverefraction in the connection between distal and proximal sections. Forexample, in place of a cone endface, a hyperbolic or spherical endfacemay be utilized to achieve desirable effects. However, the degree oflight control and simplicity of construction may suffer by utilizingnon-conical surfaces.

The present invention has been described in relation to particularembodiments which are intended in all respects to be illustrative ratherthan restrictive. Alternative embodiments will become apparent to thoseskilled in the art to which the present invention pertains withoutdeparting from its spirit and scope. For example, by utilizing twocone-shaped endfaces, one on the proximal and distal fibers, therefraction can be distributed between the two surfaces. Accordingly, thescope of the present invention is defined by the appended claims ratherthan the foregoing discussion.

What is claimed is:
 1. An optical fiber assembly comprising:a firstoptical fiber having a first ability to transmit light; a second opticalfiber having a second ability to transmit light, the second opticalfiber having the ability to transmit light having a greater angularorientation than the first fiber; and means for transmitting lightbetween the first optical fiber and the second optical fiber and foraltering the angular orientation of light between the first fiber andthe second fiber.
 2. The optical fiber assembly of claim 1, wherein thesecond fiber's ability to transmit light having a greater angularorientation results from metalization applied to the second fiber. 3.The optical fiber assembly of claim 2, wherein the metalization isapplied to the core of the second fiber.
 4. The optical fiber assemblyof claim 1, wherein the means for transmitting light between the firstoptical fiber and the second optical fiber and for altering the angularorientation of light comprises a shaped endface.
 5. The optical fiberassembly of claim 4, wherein the shaped endface is cone shaped.
 6. Theoptical fiber assembly of claim 1, further comprising a filter betweenthe first fiber and the second fiber.
 7. The optical fiber assembly ofclaim 6, wherein the filter adheres to a fiber endface.
 8. The opticalfiber assembly of claim 1, wherein the second fiber's ability totransmit light having a greater angular orientation is determined by thesecond fiber's material.
 9. The optical fiber assembly of claim 1,wherein the second fiber's ability to transmit light having a greaterangular orientation is determined by the second fiber's construction.10. The optical fiber assembly of claim 1, wherein the metalization isapplied to the cladding of the second fiber.
 11. A fiber optic probeassembly comprising:a housing having an opening at an end; a windowmounted in the opening at the end, the window sealing the end of thehousing; an optical fiber mounted in the housing, the fiber having acone-shaped endface adjacent the window; and a material filling thespace between the fiber and the window, the material having a refractiveindex suitable for controlling the pattern of light transmitted into orout of the fiber.
 12. The fiber optic assembly of claim 11, wherein thehousing comprises a cylindrical shape and the window is transparent. 13.The fiber optic assembly of claim 11, wherein at least a portion of theoptical fiber is secured within the housing and extends along alongitudinal axis of the housing.
 14. The fiber optic assembly of claim11, wherein the optical fiber is secured to the housing by a bondingagent.
 15. An optical fiber assembly for receiving or transmittingluminous energy, said optical fiber assembly comprising:a first opticalfiber having a cone-shaped endface; a second optical fiber having atleast one endface; a connector for connecting the cone-shaped endface ofsaid first optical fiber to the endface of said second optical fiber;wherein said cone-shaped endface of said first optical fiber is formedso as to increase the number of modes that are capable of propagating insaid second optical fiber.
 16. The optical fiber assembly of claim 15,wherein said shaped endface comprises a cone-shaped endface.
 17. Theoptical fiber assembly of claim 15, wherein said first fiber and saidsecond fiber comprise different materials.
 18. An optical fiber assemblycomprising:a first fiber having a proximal end and a distal end; asecond fiber having a distal end; and a connector for connecting thefirst fiber to the second fiber along their longitudinal axes, theproximal end of the first fiber being adjacent the distal end of thesecond fiber, wherein a selected one of the proximal end of the firstfiber and the distal end of the second fiber comprises a shaped endfacefor controlling the pattern of light transmitted between the first andsecond fibers.
 19. The optical fiber assembly of claim 18, wherein theshaped endface comprises a conical shape.
 20. The optical fiber assemblyof claim 18, wherein the shaped endface comprises a hyperbolic shape.21. The optical fiber assembly of claim 18, wherein the shaped endfacecomprises a spherical shape.
 22. The optical fiber assembly of claim 18,wherein the distal end of the second fiber comprises the shaped endfaceand the proximal and distal ends of the first fiber are flat.
 23. Theoptical fiber assembly of claim 18, wherein the proximal end of thefirst fiber comprises the shaped endface and the distal ends of thefirst and second fibers are flat.
 24. The optical fiber assembly ofclaim 18, further comprising a filter located between the first fiberand the second fiber.
 25. The optical fiber assembly of claim 24,wherein the filter comprises a filtering wafer.
 26. The optical fiberassembly of claim 24, wherein the filter comprises a filtering coatingapplied to the proximal end of the first fiber.
 27. The optical fiberassembly of claim 24, wherein the filter comprises a filtering coatingapplied to the distal end of the second fiber.
 28. The optical fiberassembly of claim 18, further comprising a coating applied to an outerportion of the first fiber, the coating inhibiting the passage of lightout of the first fiber.
 29. The optical fiber assembly of claim 28,wherein the coating is metallic.
 30. The optical fiber assembly of claim28, wherein the coating is applied to the core of the first fiber. 31.The optical fiber assembly of claim 28, wherein the coating is appliedto a cladding of the first fiber.
 32. The optical fiber assembly ofclaim 18, further comprising a material in a space between the first andsecond fibers, the material having a known refractive index suitable foraltering the characteristics of light traveling between the first fiberand the second fiber.
 33. A fiber optic assembly with a protectedendface, comprising:an optical fiber having a cone-shaped endface at adistal end; and a housing having a distal end and surrounding thecone-shaped endface to provide mechanical protection of the cone-shapedendface.
 34. The fiber optic assembly of claim 33, wherein the distalend of the optical fiber is even with the distal end of the housing. 35.The fiber optic assembly of claim 33, wherein the distal end of theoptical fiber is recessed relative to the distal end of the housing. 36.A fiber optic assembly, comprising:an optical fiber having a shapedendface at a distal end; and a housing having an opening at a distalend, the housing surrounding at least a portion of the shaped endface toprovide mechanical protection of the shaped endface, the openingexposing the shaped endface to the operating environment of the fiberoptic assembly wherein the shaped endface is not contained within aplane perpendicular to a longitudinal axis of the optical fiber.
 37. Thefiber optic assembly of claim 36, wherein the distal end of the opticalfiber extends even with the distal end of the housing.
 38. The fiberoptic assembly of claim 37, wherein the shaped endface extends even withthe opening of the housing.
 39. The fiber optic assembly of claim 36,wherein the distal end of the optical fiber is recessed relative to thedistal end of the housing.
 40. The fiber optic assembly of claim 36,wherein the shaped end of the optical fiber is recessed relative to theopening of the housing.
 41. The fiber optic assembly of claim 36,wherein the optical fiber is positioned along a longitudinal axis withinthe housing and the shaped endface extends even with the opening of thehousing.
 42. The fiber optic assembly of claims 36, wherein at least aportion of the optical fiber is secured within the housing and extendsalong a longitudinal axis of the housing.
 43. The fiber optic assemblyof claim 36, wherein the optical fiber is secured to the housing by abonding agent.
 44. The fiber optic assembly of claim 36, wherein thehousing comprises a rigid sleeve having a tubular shape, the openingpositioned at one end of the sleeve.
 45. The fiber optic assembly ofclaim 36, wherein the shaped endface comprises a conical shape.
 46. Thefiber optic assembly of claim 36, wherein the shaped endface comprises ahyperbolic shape.
 47. The fiber optic assembly of claim 36, wherein theshaped endface comprises a spherical shape.
 48. The fiber optic assemblyof claim 36, wherein the housing comprises a rigid sleeve having asleeve end, the opening of the housing positioned at the sleeve end. 49.A method for parting and rejoining a group of optical fibers in closeproximity to one another, comprising the steps of:bonding the opticalfibers together such that the optical fibers are fixed and constrainedtogether within a longitudinal section; parting the bound optical fiberswithin the constrained longitudinal section, each of the parted boundoptical fibers forming a first section and a second section; providing amechanical means for realigning the parted bound optical fibers; andrejoining the optical fibers together as a group such that the firstsection and second section of each bound optical fiber are aligned witheach other.
 50. The method of claim 49, wherein the step of parting thebound optical fibers comprises cutting the fibers perpendicular to theirlongitudinal axes.
 51. The method of claim 49, wherein the step ofbonding the optical fibers together comprises applying an epoxy to theoptical fibers.
 52. The method of claim 49, wherein the step of bondingthe optical fibers together comprises placing a shrinkable tubing aroundthe optical fibers.